In recent years, much importance has been given to measures to cope with environmental problems and resource problems, and development of fuel cells has been active as one of such measures. In particular, direct methanol type fuel cells, which directly utilize methanol as fuel to generate electricity without reforming or gasification, are simple in structure and can be easily reduced in size and weight. Hence they have been attracting much attention as power sources of portable small-sized electronic equipment, computer power sources and portable power sources.
A direct methanol type fuel cell comprises a cell stack of a plurality of unit cells stacked, the unit cell comprising an anode, a cathode and an electrolyte between them and having means for supplying a methanol aqueous solution as liquid fuel to the anode and means for supplying an oxidant gas such as air to the cathode. In this cell stack, separators, having channels and manifolds for supplying a methanol aqueous solution to the anodes of respective unit cells, supplying an oxidant gas to the cathodes and discharging reaction products formed by electrochemical reactions of the respective unit cells, are interposed between respective unit cells.
To put it more specifically, the channels and manifolds of the separators play roles of supplying a methanol aqueous solution to the anodes, supplying an oxidant gas to the cathodes and discharging carbon dioxide formed from the anodes and water formed from the cathodes.
When a methanol aqueous solution is fed to the anodes of such a direct methanol type fuel cell and an oxidant gas is fed to the cathodes thereof, at the anodes methanol and water will react together to form carbon dioxide, and hydrogen ions and electrons will be emitted. At the cathodes the oxidant gas will take in the hydrogen ions and electrons that permeated the electrolyte to form water. In this way, electrical energy is obtained from an external circuit.
In the above-mentioned direct methanol type fuel cell, it is desirable to raise the concentration of the liquid fuel from the viewpoint of its output characteristics. However, if the concentration is raised, the amount of methanol that permeates (crossover) the electrolyte consisting of proton-conductive polymer membrane will increase. It, therefore, was necessary to set the output characteristics by considering the drop in efficiency due to the increase in the amount of permeation of methanol. In other words, the above-mentioned direct methanol type fuel cell had a characteristic that its output characteristics and efficiency depend heavily on the operating conditions such as service temperature and feeding rates of the fuel and the oxidant gas.
Some direct methanol type fuel cells having a structure wherein a methanol aqueous solution is fed in optimum conditions to the fuel electrode have been known to impose less restraints. For example, Japanese Patent Unexamined Publication Hei 11-510311 (International Publication Number WO97/21256) proposes a structure wherein unused fuel that permeated from the anode to the cathode and carbon dioxide formed at the anode are separated from each other, the unused fuel separated and water formed at the cathode are mixed together, and while monitoring with a concentration sensor to adjust the concentration to an optimum value, methanol or water is added from a pure methanol tank or a water tank by a liquid pump. On the other hand, Japanese Patent Unexamined Publication 2000-21426 discloses a means wherein water and carbon dioxide formed by electrochemical reactions are fed into a mixer to make them react with water stored in advance in the mixer to form carbonic acid, then the carbonic acid and methanol as fuel are mixed together to effectively utilize the reaction products and restrain the drop in the utilization efficiency of the fuel. Furthermore, Japanese Patent Unexamined Publication Hei 9-161810 discloses a means wherein water and carbon dioxide that are formed by electrochemical reactions and unused methanol from which carbon dioxide has been removed are circulated by a pump while they are controlled at an optimal concentration.
In recent years, such direct methanol type fuel cells have been installed or stored as distributed electric power sources indoors and outdoors, and when the site was in a cold region, it was absolutely necessary to ensure its smooth startup. In other Words, it was vital to ensure smooth startup even when the fuel cell was installed in an environment of temperature of −20° C.